A. Literature
The open literature, which presents issues relevant to imaging systems in general, includes the following documents which are incorporated herein by reference:
B. Analog and Hybrid (Analog-Digital) Beamformer Systems
Relevant analog and hybrid (analog-digital) phased array beamformer system art can be found in the following patents which are incorporated herein by reference:
______________________________________ U.S. Pat. No.: Title: Inventor(s): ______________________________________ 4,140,022 MULTIPLE Samuel H. Maslak TRANSDUCER ACOUSTIC IMAGING APPARATUS 4,550,607 PHASED ARRAY Samuel H. Maslak ACOUSTIC IMAGING J. Nelson Wright SYSTEM 4,699,009 DYNAMICALLY Samuel H. Maslak FOCUSED LINEAR Hugh G. Larsen PHASED ARRAY ACOUSTIC IMAGING SYSTEM 5,014,710 STEERED LINEAR Samuel H. Maslak and COLOR DOPPLER Donald J. Burch 5,165,413 IMAGING J. Nelson Wright Hugh G. Larson Donald R. Langdon Joel S. Chaffin Grant Fash, III ______________________________________
C. Digital Beamformer Systems
The concept of a digital beamformer system has been proposed in the art with respect to ultrasound systems. By way of example, the following U.S. patents, all of which are incorporated herein by reference, discuss various aspects of such systems. The patents include:
______________________________________ U.S. Pat. No.: Title: Inventor(s): ______________________________________ 4,809,184 METHOD AND Matthew O'Donnell APPARATUS FOR Mark Magrane FULLY DIGITAL BEAM FORMATION IN A PHASED ARRAY COHERENT IMAGING SYSTEM 4,839,652 METHOD AND Matthew O'Donnell APPARATUS FOR HIGH William E. Engeler SPEED DIGITAL Thomas L. Vogelsong PHASED ARRAY Steven G. Karr COHERENT IMAGING Sharbel E. Noujaim SYSTEM 4,886,069 METHOD OF, AND Matthew O'Donnell APPARATUS FOR, OBTAINING A PLURALITY OF DIFFERENT RETURN ENERGY IMAGING BEAMS RESPONSIVE TO A SINGLE EXCITATION EVENT 4,893,284 CALIBRATION OF Mark G. Magrane PHASED ARRAY ULTRASOUND PROBE 4,896,287 CORDIC COMPLEX Matthew O'Donnell MULTIPLIER William E. Engeler 4,975,885 DIGITAL INPUT STAGE Dietrich Hassler FOR AN ULTRASOUND Erhard Schmidt APPARATUS Peter Wegener 4,983,970 METHOD AND Matthew O'Donnell APPARATUS FOR William E. Engeler DIGITAL PHASED John J. Bloomer ARRAY IMAGING John T. Pedicone 5,005,419 METHOD AND Matthew O'Donnell APPARATUS FOR Kenneth B. Welles, II COHERENT IMAGING Carl R. Crawford SYSTEM Norbert J. Plec Steven G. Karr 5,111,695 DYNAMIC PHASE William E. Engeler FOCUS FOR COHERENT Matthew O'Donnell IMAGING BEAM John T. Pedicone FORMATION John J. Bloomer 5,142,649 ULTRASONIC IMAGING Matthew O'Donnell SYSTEM WITH MULTIPLE, DYNAMICALLY FOCUSED TRANSMIT BEAMS 5,230,340 ULTRASOUND Theador L. Rhyne IMAGING SYSTEM WITH IMPROVED DYNAMIC FOCUSING 5,235,982 DYNAMIC TRANSMIT Matthew O'Donnell FOCUSING OF A STEERED ULTRASONIC BEAM 5,249,578 ULTRASOUND Sidney M. Karp IMAGING SYSTEM Raymond A. Beaudin USING FINITE IMPULSE RESPONSE DIGITAL CLUTTER FILTER WITH FORWARD AND REVERSE COEFFICIENTS ______________________________________
D. Adjustable Frequency Scanning
Ultrasound medical systems using phased arrays have been used for some time. Three basic scan and display formats have generally been used in combination with planar linear transducer arrays, that is, arrays in which the transmit/receive surface of individual transducer elements are positioned in a single plane (approximately) and generally have uniform element spacing.
Two-dimensional images have been formed by a linear-type scan format where ultrasonic beams corresponding to parallel scan lines normal to or at a constant angle to a line connecting the transmit surfaces of the elements are generated by selected groups of transducer elements shifted across the array. Linear scanning with parallel scan lines has a field of view determined by the width of the physical aperture of the transducer array.
In a sector-type scan format, the transducer elements are spaced much closer together than generally used for linear scan transducers, typically at half-wavelength or so intervals. This permits acoustic scan lines to be steered without generating grating lobes and allows both the size of the transducer array to be decreased and the field of view to be increased at deeper depths. Sector-phased arrays form acoustic scan lines that all originate from a single point at the center of the face of the transducer array.
In a Vector.RTM.-type scan format, the scan lines lie along rays which need not originate from a single point on the face of the transducer array, but rather may pass through a substantially common vertex that is typically not on the face of the transducer array. The variably located vertex is usually a selectable distance behind the face of the transducer array to provide an extended field of view. The common vertex can be anywhere (including in front of the array) and need not be on a centerline normal to the array. More generally, the individual scan lines in a Vector.RTM. format scan can intersect the transmitting surface of the array at different origins and with different steering angles relative to a normal to the array. The Vector.RTM. scan format is described in U.S. Pat. Nos. 5,148,810; 5,235,986; and 5,261,408, and are incorporated herein by reference.
The linear, sector and Vector.RTM. scan formats can be used also with transducer arrays whose transmitting surfaces are not planar, but rather are curved. In addition, many scan formats can be defined other than linear, sector and Vector.RTM. formats, but all can be defined by specifying, for each scan line, both a point of intersection with the transducer array and a steering angle relative to a respective normal thereto, and the minimum and maximum range of the scan line. For three-dimensional scan formats, another parameter set may need to be specified as well such as using two steering angles (azimuth and elevation) relative to a respective normal. Also, while the transmitting and receiving arrays are typically the same physical array, in general they can be different physical arrays.
It is well-known that phased array ultrasonic imaging with a sampled aperture is subject to the effects of grating lobes due to the periodic nature of the array element spacing and the coherent nature of the ultrasound waves. Grating lobes appear as regions of sensitivity at angles away from the direction of interest, and can produce interference effects resulting in image ambiguity artifacts.
High spatial resolution and a large field of view are desirable qualities for an ultrasound imaging system. The former requires high frequency operation and/or a large active aperture, and the latter requires a large imaging aperture and/or the ability to steer the imaging beam far from the normal to the transducer without incurring grating lobe artifacts.
When phased array imaging beams are steered far away from the normal, i.e. 30 degrees or more, the appearance of grating lobes may become a more significant contributor to image artifacts. Further, sensitivity or gain is lost due to the effects of loss of sensitivity of the individual elements at high steering angles. In fact, the combination of these two effects act together to worsen the impact of grating lobe artifacts.
One means for countering the effects of reduced sensitivity from steered beams is to lower the nominal imaging center frequency. This will, however, reduce the overall spatial lateral resolution in the image field of view if applied to every scan line. Another means is to increase the system gain for beams away from the center of the image. But this is merely a gain adjustment, and does nothing to improve signal-to-noise ratio or dynamic range.
Other means for mitigating the effects of grating lobes involve disadvantageous compromises. Reducing the element-to-element spacing, typically to half-wavelength spacing at the center frequency, will limit or eliminate the grating lobe artifact, but does so at the expense of lateral resolution. Increasing the number of active electronic channels may also help, but at a substantial increase in complexity and cost. Reducing the maximum steering angle can also mitigate the grating lobe artifact, but at the expense of a narrowed field of view.
Means for reducing the grating lobe by means of decreasing the imaging frequency on a scan-line-by-scan-line basis has previously been described in Yamaguchi, et al., U.S. Pat. No. 4,631,710. In this patent, a variable bandpass filter is described whose characteristics (high and low cut-off frequencies) are controlled as a function of steering angle. This filter includes a wideband amplifier, a variable high pass filter, and a variable low pass filter. It works by cutting off high frequency pulse energy selectively with scan angle. As described, this filter may be applied to some combination of: (1) each output from a pulser, or transmitter, (2) each reception signal from a receiving transducer element, (3) a composite receive beam, (4) an original transmission or excitation signal.
The above means to control grating lobe energy suffers from many disadvantages. First, providing one such device per transmit channel and/or one per receive channel would be prohibitively complex and electronic component intensive for a high performance ultrasound imaging system, wherein 128 or more transmitters and receivers may be active simultaneously, thus requiring 256 or more such devices. These devices would have to comprise extremely well-matched, tight tolerance parts over a very broad frequency range, since they are being applied pre-beamforming, in order to avoid introducing phase errors that could otherwise destroy the coherence needed for beamformation summation. All of this means that the added cost of building individual bandpass filters on a per channel basis for either receive, transmit, or both, would be very high.
Secondly, the proposal to apply the variable bandpass filter to an original transmission signal would require that an arbitrary waveform be delayed on a per channel basis after wave shaping. Not only does this involve a dedicated analog delay line for each active transmit channel, but each delay line would have to possess a very large bandwidth because the center frequency of each excitation pulse must vary as a function of scan line over a large frequency range, even for a particular transducer. Of course, if the imaging system is to be used for a variety of transducers with widely varying center frequencies (typical transducer center frequencies range from 2.0 MHz to 7.5 MHz or more for modern high performance medical ultrasound imaging systems), the bandwidth requirement becomes extreme. For a high performance ultrasound imaging system, which typically has from 32 to 128 or so simultaneously active transmitters, this again represents very high cost.
Thirdly, the approach of the Yamaguchi patent suffers from an inherent loss of signal-to-noise ratio because useful signal energy is lost. This applies generally, regardless of where in the system the variable bandpass filter is applied. Concentrating on the patent's proposal to apply the variable bandpass filter to the composite receive beam (since all remaining proposals, as described above, are not realistically realizable with reasonable cost, other considerations notwithstanding), the approach taken in the patent amounts to simply discarding signal energy received outside the band of interest. Thus, if the bandwidth is reduced by 50%, then 50% of the signal energy is simply discarded. Moreover, the signal energy being discarded is in precisely the frequency region where the transducer is maximally sensitive, and so a very significant loss in signal-to-noise ratio results.
Grating lobe reduction is discussed also in ELECTRONIC LETTERS, Jul. 4, 1991, vol. 27, no. 14, "Grating Lobe Reduction in Ultrasonic Synthetic Focusing", by M. H. Bae, I. H. Sohn, and S. B. Park. However, the solution discussed therein applies only to synthetic focusing with a single element. The solution amounts to effectively reducing the "spacing" between adjacent synthesized elements by a factor of two. In array processing, this amounts to increasing the number of simultaneously active channels by a factor of 2, which is already well known to the art and does not solve the problem of reducing grating lobes with a given number of channels.